Combustion of carbon containing fuels, for example fossil fuels, can produce gaseous emissions of CO2 and other pollutants, for example nitrogen oxides (individually “NO”, “NO2”, and other nitrogen oxides; collectively “NOx”). CO2 has been identified as a “greenhouse” gas which may contribute to global warming. Technologies have been developed to inhibit large quantities of CO2 from being released into the atmosphere during the combustion of fossil fuels. Such technologies may be employed, for example, in coal fired power plants to reduce CO2 in flue gases generated during combustion. The technologies generally fall into one of three areas: post combustion capture, where CO2 is removed after combustion; pre-combustion capture, where the CO2 is removed before combustion, and oxy-firing combustion, where the fuel is burned in pure oxygen or oxygen enriched gas instead of atmospheric air.
Oxy-firing combustion provides a concentrated stream of CO2, compared to air-firing combustion, which facilitates capture and storage of CO2. By completing combustion in pure oxygen or oxygen enriched gas, the resultant flue gas leaving the combustion chamber generally has less non-reactive constituents, such as nitrogen, compared to a flue gas leaving an air-firing combustion chamber.
Prior to removing CO2 from a flue gas stream of an oxy-firing combustion system, the flue gas stream may be subjected to one or more techniques to remove contaminants, for example NOx, produced during combustion. For example, certain control systems may reduce NOx or inhibit its formation during combustion by adjusting certain variables in the combustion process, including but not limited to, the combustion temperature, the flow rate of fuel into the combustion chamber, and/or the flow rate of combustion gas(es) into the combustion chamber. A disadvantage of such control systems is that they can affect the efficiency of the combustion process. An alternative approach is to limit NOx emissions by re-circulating the flue gases generated during combustion into the combustion chamber.
Accordingly, there is need for an improved method and system for removing NOx from a flue gas of an oxy-firing combustion system.